Low noise electron emitters



Sept. 27, 1966 w, FEIST 3,275,869

LOW NOISE ELECTRON EMITTERS Filed July 12, 1963 2 Sheets-Sheet llNVE/VTOR WOLFGANG M. FE/ST l B) W ATTORNEY Sept. 27, 1966 w. M. FElST3,275,869

LOW NOISE ELECTRON EMITTERS Filed July 12, 1963 2 Sheets-Sheet 2 I I 11I m m E I i VACUUM LEvEL 0 5 cououcnou BAND z I I METAL FORBIDDEN ZONE SBACKING EMPTY ACCEPTOR P'TYPE SURFACE 2 |(STATE$ I1 STATES '5 -L FERMILEvEL LL! 3 VALENCE BAND THICKNESS OF SEMICONDUCTIVE LAYER MEASURED FROMsAcK|-s- T l I Q I I 11 z m o I {5 i H E 1 1- VACUUM LEvEL 2 m FORBIDDENZONE METAL l s N TYPE SURFACE BACKING EMPTY ACCEPTOR g STATES STATES EFERM| LEvEL LU 5 Z/ QQQ QV OCCgLErISACCEPTOR o THICKNESS OFSEMICONDUCTIVE LAYER MEASURED FROM BACKlNG- INVENTO/P WOLFGANG M. FE/STATTORNEY United States; Patent 3,275,869 LOW NOISE ELECTRON EMITTERSWolfgang M. Feist, Burlington, Mass, assignor to Raytheon Company,Lexington, Mass., a corporation of Delaware Filed July 12, 1963, Ser.No. 294,555 8 Claims. (Cl. 31365) The present invention is concernedwith an electron emissive device and, more particularly, with anelectron emissive device for producing a stream of electrons having anarrow range of energies.

While the invention may be applied to electron tubes in general, it isparticularly useful in a travelling wave tube, an electron tube in whichan electron beam of very low noise content is especially desirable.

Presently, the large majority of electron tubes utilize thermioniccathodes wherein a stream of electrons is emitted by heating a cathodeto high temperatures (e.g. 1500 K.). The kinetic energiesor velocitiesof electrons produced by thermionic emission vary over a wide range ofvalues which, theoretically has no upper limit, i.e. a Maxwelliandistribution. Such a wide spread of electron energies manifests itselfas undesirable noise in the output signal of a travelling wave tube orsimilar device.

It is therefore an object of the present invention to provide a lownoise source of electrons which is suitable for use in a variety ofelectron tubes, including travelling wave tubes.

A low noise electron emitter constructed in accordance with the presentinvention comprises a cathode having a surface layer of material whichresponds to electromagnetic radiation by emitting electrons. Thematerial is substantially insensitive to radiation at wave-lengths abovea threshold wavelength A The electron emitter further comprises a sourceof electromagnetic radiation aligned with the cathode surface, theradiation from the source extending over a range of wavelengthsbeginning at a lower wavelength limit h which is less than, but closeto, the threshold wavelength i of the cathode surface material.

In accordance with a further aspect of the invention, means are alsoprovided for cooling the cathode to temperatures substantially belowroom temperature so as to aid in reducing the energy spread of electronsemitted from the cathode.

The invention will now be further described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic representation of a first embodiment of theinvention as applied to a travelling wave tube;

FIG. 2 is a schematic representation of a second em bodiment of theinvention as applied to a travelling wave tube;

FIG. 3 is an energy level diagram of a cathode having a semiconductiveelectron emitting layer with p-type surface states which may be employedin the FIG. 1 or FIG. 2 embodiments of the invention; and

FIG. 4 is an energy level diagram of a cathode having a semiconductiveelectron emitting layer with n-type surface states which may be employedin the FIG. 1 or FIG. 2 embodiments of the invention.

Referring to FIG. 1, a schematic representation of a travelling wavetube, indicated generally by the reference numeral 10, comprises anevacuated glass envelope 11, a slow wave structure shown as a helix 12,a collector electrode 13 and an electron emissive cathode 14. Additionalelements of a conventional travelling wave tube such as the input andoutput waveguides, the electron accelerating anode and electric andmagnetic beam focusing means have been omitted since they are notessential to an understanding of this invention. In accordance with theinvention, cathode 14 is sensitive to electromagnetic radiation over agiven range of wavelengths, beginning at a threshold wavelength A foremitting electrons. While the invention is not restricted to cathodessensitive to radiation at wavelengths in the visible spectrum, theinvention will be further described with reference to such a cathodesince materials sensitive to radiation at such Wavelengths are wellknown. Cathode 14 may comprise either one of the commonly knownphotoemissive metals or, as will be described hereinafter, cathode 14comprises an electron emitting layer of semiconductive material 14aadhered to a metallic backing layer 14b. Preferably, emitting layer 14acomprises a layer of an alkali antimonide such as the intermetalliccompound of cesium and antimony 05 8b to which additional p-typeacceptor impurities have been added as w ll lbe more fully explainedbelow in connection with FIGS. 3 and 4.

A source of electromagnetic radiation, indicated generally by referencenumeral 15, is disposed external to envelope 11 and optically alignedwith emitting layer 14a of cathode 14. Source 15 comprises a lightsource 16, a reflector 17, a low-pass filter 18 and a focusing lens 19.Low-pass filter 18 serves to block radiation at wave-lengths less than alimiting wavelength R where A is close to but less than the thresholdwavelength A of cathode 14. From the standpoint of efficiency, the upperwavelength limit of source 16 isalso preferably close to A However, thislatter limit is not essential to the invention. Radiation source 15 may,in the alternative, comprise any other type of device which providesradiation having a sharply defined lower wavelength limit. For example,an essentially monochromatic source of radiation such as a gaseousdischarge device or a laser may be used to provide the desired radiationor illumination. 7

A means for cooling cathode 14, shown as a refrigerating enclosure 20,is disposed around the outside of glass envelope 11 at the end thereofwhich contains cathode 14. An inlet conduit 21 and an outlet conduit 22are attached to refrigerating enclosure 20 to permit entry and dischargeof a cooling medium. Refrigerating enclosure 20, or at least that partthereof which lies along the optical path between cathode 14 andradiation source 15, is transparent to radiation at wavelengths betweenthe lower wavelength limit h of source 15 and the threshold wavelength tof cathode 14.

Referring now to FIG. 3 of the drawinggan energy level diagram is shownfor a cathode having an electron emitting layer of semiconductivematerial constructed in accordance with the invention. 'Electron energyis plotted along the axis of ordinates, while the thickness ofsemiconductive layer 14a measured from metallic backing 14b is plottedalong the of abscissae. V

In the diagram of FIG. 3, the metallic backing layer 14!) of cathode 14is shown to the left of the axis of ordinates, while the emitting layerof semiconductive material 14a is shown to the right of the axis ofordinates. The semiconductive layer is divided into three regions inFIG. 3. The three regions are designated as the region of contactbetween the metallic backing and the semiconductive layer (I), the bulkregion (II) and the surface region (III). Furthermore, three discreteenergy level bands are shown, the highest being the conduction bandwhich defines the energy levels normally occupied by some conductionelectrons when the material is at temperatures above absolute zero i.e.greater than 0 K.), the lowest being the valence band which defines theenergy levels occupied by the more tightly bound valence electrons andthe intermediate being the forbidden energy gap or zone which separatesthe allowable valence and conduction electron energy levels. The Fermilevel is indicated by a dashed line and is that energy level at whichthe probability of occupation by an electron is /2 A plurality ofunoccupied acceptor states which may, for example, be produced bydiffusion of excess cesium atoms or other p-type impurities within thesemiconductive layer 14a, are arranged at an energy level in theforbidden zone close to the upper limit of the valence band. Thepresence of unoccupied acceptor states close to the valence band servesto shift the Fermi level of the material from a level approximatelymidway in the forbidden gap to a level close .to the upper. limit of thevalence band. A shift of the Fermi level of this nature is indicative ofthe fact that fewer electrons are available for emission from energylevels above the valence band when such acceptor states are present.

wavelength A of emitting layer 14a aswill be more fully explained below.Light impinging upon emitting layer 14a causes electrons to be emittedfrom layer 14a towards helix 12 and collector 13. In accordance with thewell-known operation of a travelling wave tube, the electrons arefocused into a beam by focusing fields (not shown). The beam ofelectrons passes through the center of helix 12, giving up energy toelectromagnetic waves which are coupled to helix 12 by means of awaveguide or similar; device (not shown). The amplified electromagneticwave is coupled out of helix 12 by means of a second waveguide (notshown) and the electrons are collected by collector 13. 7

The emission of electrons from cathode 14 will now be consideredingreater detail. The maximum energy or velocity of the electrons'emitted from emitting layer 14a depends upon (1) The wavelength (orfrequency) of the incident light, and

(2) The difference between the potential energy of the electron withinthe layer 14a and the potential en ergy of .an electron at the vacuumlevel outside the layer 14a .(the latter difference being thephotoelectric work function where the emitted electron occupied thehlghest energy level occupied by electrons within the layer 14a).

Specifically, the maximum energy of an emitted electron is given by theexpression The threshold wavelength t of emitting layer 14a is 7 relatedto the work function thereof by theexpression where A is measured inthousands of Angstrom units and is measured in electron volts.

Furthermore, the maximum energy difference between photo-emittedelectrons is given by the expression Furthermore, the lower wavelengthlimit M is close to but less than the threshold.

' where It can therefore be seen that the maximum. energy differencebetween photo-emitted. electrons may be controlled for a material havinga given threshold wavelength by limiting the minimum wavelength of theincident light to a value close to the threshold wavelength.

A In a travelling'wave tube such as is shown ind- IG- 1, it is possible,in'accordance .with the present invention, to restrict the maximumenergy. spread of emitted electrons to values not presently attainablefrom a thermionic cathode itself. For example, in order .to restrict themaximum energy spread of emitted electrons to a value of 0.1 ev.(electron volts),the lower wavelength limit h of source 15 is restrictedto a wavelength which where a and )\11 are measured in thousands ofAngstrom units.

ures, of the order of 7000 A. (Angstrom units), a case, the aboveexpression is satisfied ifthe lower frequency limit h of source 15 lies.within the. limits Further reduction in the energy spread betweenemitted electrons may be achieved by further limiting the lowerwavelength limit h of source 15.

In order to achieve practical levels of emission from a cathodeilluminated by such a narrow band of wave- 7 lengths near the thresholdwavelength A th photoelectric yield (i.'e. .unit of emission current perunit of incident illumination) of emitting layer must rise sharplywithin the range of wavelengthsbetween A and R The desired sharplyrising photoelectric yield characteristicis obtained by insuring that alarge number ofelectrons' occupy energy levels close to the highestoccupied energy level in emitting layer14a. ,Sirice vthemajority ofelectrons within any material occupy energy. levels in the valence bandwhile only relatively. few electrons are thermally excited to higherenergy levels (e.g. in the con duction band or, as with p-typesemiconductors, in

defect level states in the forbidden gap), the desired large a number ofelectrons Within a narrow band of energies is best obtained bypreventing electrons from occupying levels above the valence band.Ideally, the photoelectric work function will. then be equal-to. thedifference flfitW6l1 the valence .band upper-limit and the vacuum evel.

tained (1). By cooling'cathode 14 by means of refrigerating" enclosure20 so as to freeze. electronsout of energy; level states above thevalence band, and

(2) By introducing p-type acceptor impurities having unoccupied energylevels near. the upper limit of 1 the;

valence band throughout emitting layer 14a.

A cooling medium such :as a liquified gas (e.g. helium) is passedthrough refrigerating enclosure..20 by means of inlet conduit 21 andoutletconduit 22.80 as to cool cathode 14 to a temperature substantiallybelow room temperature (e.g. even approaching absolute zero) and fthereby reduce the number of electrons occupying energy levels above thevalence. band energy levels. a

The threshold wavelength A for a cathode having a cesium-antimonysurface may. be,'in round fig- In such Furthermore, the photo-emittedelectrons will be supplied almost exclusively ,fromvalence bandenergylevels. In accordance with the invention, the. desired sharply risingphotoelectric yield over a'narrow. band of 1 wavelengths close to thethreshold wavelength t is ob-1.

The unoccupied p-type acceptor states (shown as circles in FIGS. 3 and4'above the Fermi level) serve to decrease the probability that energylevels above the valence band are occupied by electrons. Suflicientacceptor states may even be added to depress the Fermi level below theupper limit of the valence band.

Referring now to FIG. 2, a second embodiment of the invention in atravelling wave tube is shown. Corresponding parts in FIG. 2 areindicated by the same reference numerals as in FIG. 1.

The FIG. 2 embodiment is generally similar to the FIG. 1 embodiment withthe exception that cathode 14 comprises an electron emitting layer 14aand a semitransparent metallic backing layer 14c. In this case, theradiation from source '15 impinges upon semitransparent backing layer14c and passes through semitransparent backing layer 14c, causing theemission of electrons from emitting layer 14a towards helix 12. Thethickness of emitting layer 14a is preferably approximately equal to themaximum escape depth for photoexcited electrons (e.g. about 200-300Angstrom units for an alkali antim-onide) so as to obtain a highphotoelectric yield from cathode 14. In all other respects, theembodiment shown in FIG. 2 corresponds to that shown in FIG. 1.

Referring now to FIG. 4, an energy level diagram similar to the energylevel diagram of FIG. 3 is shown. However, FIG. 3 indicates the effecton the energy level diagram of the presence of p-type surface stateswhereas FIG. 4 indicates the effect thereon of the presence of n-tyoesurface states. Surface states having energy levels within the forbiddengap of a semiconductor result from the presence of impurities on thesemiconductor surface. Where, as in FIG. 3, both the bulk region (regionH) and the surface states (region III) are p-type, the surface states donot have any noticeable effect on the energy level diagram. However,where, as in FIG. 4, the bulk region (H) is of p-type material and thesurface states are n-type, the upper limit of the valence band and thelower limit of the conduction band bend downward. This band bendingtakes place in the following manner. At equilibrium, the Fermi levelmust be the same within bulk region (II) as it is within the surfaceregion (111). Therefore, the presence of n-type surface states requiresa redistribution of negative charge from the surface states tounoccupied p-type acceptor states within the semiconductive material. Asshown in FIG. 4, acceptor states near the surface are filled or occupiedby electrons transferred from the n-type surface states. The electricfields set up by this redistribution of charge result in the downwardbending of the energy bands (i.e. valence and conduction bands) near thesurface of the semiconductive material. The occupied acceptor states inthe surface region (III) might shift the threshold wavelengths X towardshigher wavelengths. It is therefore desirable to minimize the effect ofsuch n-type surface states. This objective may be accomplished by eitherabsorbing a monolayer of molecules on the surface of emitting layer 14aso as to preclude the formation of n-type surface states or by providingsufiicient p-type acceptor states close to the surface so as to minimizethe distance over which the band bending occurs.

While this invention has been described with reference to cathodematerials which are sensitive to radiation at wavelengths in the visiblespectrum, it will be recognized that the invention contemplates use ofradiation sources and emitting devices which operate at otherwavelengths as well.

Furthermore, while it has been indicated that, in accor-dance with theinvention, an electron beam having a maximum energy spread of 0.1electron volts may be produced, practical beam currents (eg 20microamperes) may be produced with a maximum energy spread of, forexample, 0.02 electron volt using the type of cathode materialsdescribed above.

What is claimed is:

1. A low noise electron emitter comprising a cathode having a layer ofmaterial responsive to electromagnetic radiation for emitting electrons,said material being substantially non-responsive to radiation atwavelengths above a threshold wavelength, said electron emitter furthercomprising means for producing electromagnetic radiation over a range ofwavelengths, said means being aligned with said cathode, the shortestwavelength produced by said means being "less than said thresholdwavelength of said layer of electron emitting material and said shortestwavelength being limited with respect to said threshold wavelength tolimit the maximum energy diiference between electrons emitted from saidlayer of electron emitting material.

'2. A low noise electron emitter comprising a cathode having a layer ofmaterial responsive to electromagnetic radiation for emitting electrons,said material being substantially non-responsive to radiation atwavelengths above a threshold wavelength, said electron emitter furthercomprising means for producing electromagnetic radiation over a range ofwavelengths, said means being aligned with said cathode, the shortestwavelength produced by said means being less than said thresholdwavelength of said layer of electron emitting material and said shortestwavelength being limited with respect to said threshold wavelength tolimit the maximum energy difference between electrons emitted from saidlayer of electron emitting material, said electron emitter still furthercomprising means for cooling said cathode to reduce the differencebetween said threshold wavelength and the shortest wavelength radiationproduced by said means for producing electromagnetic radiation, wherebyelectrons having a narrow range of energies corresponding to thedifference in wavelength between said threshold wavelength and saidshortest wavelength are emitted from said cathode.

3. A low noise electron emitter comprising a cathode having a 'layer ofmaterial responsive to electromagnetic radiation for emitting electrons,said material being substantially non-responsive to radiation atwavelengths above a threshold wavelength, said electron emitter furthercomprising a source of electromagnetic radiation aligned with saidcathode, means interposed between said cathode and said source forlimiting the radiation impinging upon said cathode to a range ofwavelengths having a lower limit, the lower wavelength limit being lessthan said threshold wavelength and selected with respect to saidthreshold wavelength to limit the maximum energy difference betweenelectrons emitted from said layer of electron-emitting material, saidelectron emitter still further comprising means for cooling said cathodeto decrease the difference between said threshold wavelength and saidlower wavelength limit.

4. A low noise electron emitter comprising a cathode, said cathodecomprising a *layer of electron-emissive material responsive toelectromagnetic radiation over a given range of wavelengths for emittingelectrons, said material being substantially non-responsive to radiationabove a threshold wavelength, thethreshold, wavelength varying with thetemperature of said material, said electron emitter further comprising asource (if-electromagnetic radiation, said source having a shortwavelength limit which is within the range of wavelengths to which saidmaterial is responsive and which is close to the threshold wavelengthfor said material at a temperature of absolute zero, said electronemitter still further comprising means for directing radiation from saidsource of radiation to said cathode and means for cooling said cathodeto reduce the difference between said threshold wavelength and saidshort wavelength limit whereby electrons having a narrow range ofenergies corresponding to the diiference in wavelength between saidthreshold and said short wavelength limit are emitted from said cathode.

5. A low noise electron emitter comprising a cathode having a surface ofmaterial responsive to electromagnetic radiation for emitting electrons,said material being substantially non-responsive to radiation atwavelengths above a threshold wavelength t said emitter furthercomprising a source of electromagnetic radiation aligned with saidcathode, said source having a lower wavelength limit R which is relatedto said threshold wavelength a by the expression where R and A aremeasured in thousands of Angstrom units (A.) whereby the maximum energydifference between electrons emitted from said material is equal to orless than 0.1 electron volt.

6. A lowvnoise electron emitter comprising a cathode, said cathodecomprising a layer of semiconductive material responsive toelectromagnetic radiation over a given wavelength range for emittingelectrons, said material being substantially non-responsive to radiationat wavelengths above a threshold wavelength, said cathode furthercomprising p-type acceptor states diffused within said semiconductivematerial, the energy levels of said acceptor states being substantiallycloser to the upper limit of valence band energy levels than to thelower limit of conduction band energy levels of said semiconductivematerial, said electron emitter further. comprising a source ofelectromagnetic radiation, said source having a short wavelength limitless than said threshold wavelength of said semiconductive material'butlimited with respect to said threshold wavelength to limit the maximumenergy difference between electrons emitted from said layer ofsemiconductive material, said electron emitter still further comprisingmeans for directing radiation from said source of electromagneticradiation to said cathode and means for cooling said cathode wherebyelectrons having a narrow range of energies corresponding to thedifference in wavelength between said threshold wavelength and said 4short wavelength limit are emitted ,from' said layer of semiconductivematerial.

7. A low noise electron emitter comprising a cathode responsive toelectromagnetic radiation over a range of wavelengths beginning. at anupperthreshold Wavelength for emitting electrons, said cathodecomprising a metallic backing layer and an electron emitting layer ofsemiconductive material, one surface of said emitting layer being incontact with one surface of said backing layer,gsaid emitting layer,including diffused p-type acceptor impurities having acceptor energylevels in the vicinity of the maximumvalence band energy level of saidemitting layer, said emitting layer being substantially equal in depthto the maximum escape depth for photoexcited electrons from saidemitting layer, said electron emitter further comprising a source ofelectromagnetic radiation, the radiation having a sharply defined shortwavelength limit, said short wavelength limit being lessthansaidthreshold wavelength of said semiconductive material but limitedwith respect to'said threshold wavelength to limit the maximum energydifference between electrons emitted from said semiconductive material,said electron emitter still further comprising means for directingradiation from said source of electromagnetic radiation to said cathodeand means for cooling said cathode whereby elec.- trons having a narrowrange of energies corresponding to the difference in wavelength betweensaid threshold wavelength and said short wavelength limit are emittedfrom the cathode.

8. A low noise electron emitter comprising a cathode, said cathodecomprising a layer of semiconductive ma -r terial responsive toelectromagnetic radiation over, a given wavelength range for emittingelectrons, said material being substantially non-responsive to radiationat wave lengthsa-bove a threshold wavelength, said semiconductivematerial comprising a compound of compound alkali metals and antimonywith p-type acceptor states diffusedj within said layer ofsemiconductive material, the energy levels of said acceptor states beingsubstantially closer to the valence band energy level than to'theconduction band energy level'of said semiconductive material, 'said'electron emitter further comprising a source of electro magneticradiation alignedwith said cathode, the shortest wavelength produced bysaid source. being :less than the threshold wavelength of saidsemiconductive material and limited with respect to said thresholdwavelength to limit the maximum energy difference between electronsemitted I from said semiconductive material, said electron emitter stillfurther comprising means for cooling said cathode to a temperature inthe vicinity of absolute zerowhereby I electronshaving a narrow range ofenergies correspond ing to the difference in wavelengths between said,threshold wavelength and said shortest wavelength of said source areemitted from said semiconductive material;

References Cited by the Examiner UNITED' STATES PATENTS 3,036,2343,048,797

JOHN W. HUCKERT,Primary Examiner. J. D. CRAIG, Assistant Examiner.

5/1962 Dacey 3l3105, 8/1962 Linder 3l7--234-

1. A LOW NOISE ELECTRON EMITTER COMPRISING A CATHODE HAVING A LAYER OFMATERIAL RESPONSIVE TO ELECTROMAGNETIC RADIATION FOR EMITTING ELECTRONS,SAID MATERIAL BEING SUBSTANTIALLY NON-RESPONSIVE TO RADIATION ATWAVELENGTHS ABOVE A THRESHOLD WAVELENGTH, SAID ELECTRON EMITTER FURTHERCOMPRISING MEANS FOR PRODUCING ELECTROMAGNETIC RADIATION OVER A RANGE OFWAVELENGTHS, SAID MEANS BEING ALIGNED WITH SAID CATHODE, THE SHORTESTWAVELENGTH PRODUCED BY SAID MEANS BEING LESS THAN SAID THRESHOLDWAVELENGTH OF SAID LAYER OF ELECTRON EMITTING MATERIAL AND SAID SHORTESTWAVELENGTH BEING LIMITED WITH RESPECT TO SAID THRESHOLD WAVELENGTH TOLIMIT THE MAXIMUM ENERGY DIFFERENCE BETWEEN ELECTRONS EMITTED FROM SAIDLAYER OF ELECTRON EMITTING MATERIAL.